Optical modulators based on semiconductor multiple quantum well (MQW) or superlattice (SL) structures are known.
An MQW structure comprises a stack of thin layers of narrow bandgap semiconductor material alternating with layers of wide bandgap semiconductor material so that each layer of narrow bandgap material is sandwiched between two layers of wide bandgap material. The alternating structure forms a series of quantum wells located in the narrow bandgap layers that are capable of confining conduction band electrons and valence band holes. Each narrow bandgap layer has a quantum well that confines conduction band electrons and a quantum well that confines holes in the valence band.
Width of the quantum wells in a narrow bandgap layer is substantially equal to the thickness of the narrow band layer. Generally, thickness of the narrow bandgap layers, and as a result, width of the quantum wells is substantially less than a diameter of an exciton that may be generated as an intermediate state when a photon excites an electron from the valence band to the conduction band of the narrow bandgap material. Depth of the electron quantum wells is substantially equal to a difference between bottoms of the conduction bands of the wide bandgap material and the narrow bandgap material. Depth of the hole quantum wells is substantially equal to a difference between tops of the valence bands of the wide bandgap and narrow bandgap materials.
In an MQW structure, depths of the quantum wells and thickness of the wide bandgap layers are such that a wave function of an electron or hole trapped in a quantum well of a narrow bandgap layer generally extinguishes rapidly in the wide bandgap layers on either side of the narrow bandgap layer. As a result, in an MQW structure, electrons and holes confined in quantum wells of a narrow bandgap layer are substantially isolated from electrons and holes confined in other narrow bandgap layers. Hereinafter, the wide bandgap layers are referred to as “barrier layers” and the narrow bandgap layers are referred to as “quantum well layers”.
When an electric field is applied perpendicular to the planes of the layers in an MQW structure, energy levels of allowed wave functions for trapped electrons and trapped holes in the quantum wells of a same quantum well layer are displaced towards each other. As a result, a minimum amount of energy required to transfer an electron from the valence band to the conduction band and produce thereby an electron-hole pair is reduced and the absorption spectrum of the MQW is red shifted. The red shift is a result of a quantum confined Stark effect that reduces a minimum amount of energy required to excite an electron-hole exciton as an intermediate state in raising an electron from the valence band to the conducting band. Changes, on the order of 10,000 cm−1 in an absorption coefficient for light having a wavelength, hereinafter an “operating wavelength”, near an absorption edge of an absorption spectrum of an MQW structure can be realized by red shifting the absorption spectrum.
U.S. Pat. No. 4,525,687, the disclosure of which is incorporated herein by reference, describes a small aperture MQW optical shutter comprising 50 GaAs narrow band quantum well layers sandwiched between wide bandgap barrier layers formed from Ga(1-x)AlxAs with x˜0.36. The layers are formed in the intrinsic part of a pin diode. PCT Publication WO 99/40478, the disclosure of which is incorporated herein by reference, describes a wide aperture MQW high frequency optical modulator.
Performance of an MQW modulator is limited, inter alia, by an escape time for electrons and holes trapped in quantum wells of the MQW. Once holes and electrons are trapped in the MQW quantum wells of an MQW modulator after the modulator interacts with a beam of light, the electrons and holes require a finite escape time before they leave the quantum well region of the modulator. The same quantum wells that provide the modulating effects of an MQW modulator generally retard removal of the electrons and holes from the quantum wells. Buildup of photo-induced electrons and holes in quantum wells tends to shield and reduce effectiveness of electric fields applied to the MQW that are used to red shift the absorption spectrum of the modulator. In addition, current in the MQW layers generated by motion of the photo-induced electrons and holes in response to electric fields applied to the modulator can cause ohmic heating of the layers. The heating can result in an unwanted shift in the absorption edge of the modulator's absorption spectrum.
U.S. Pat. No. 5,210,428 to K. Goossen, the disclosure of which is incorporated herein by reference, notes that an article published in Applied Physics Letters, Vol. 57, No 22, pp suggests that escape times in an MQW modulator may be reduced by decreasing quantum well depth and barrier layer thickness. The patent describes an MQW modulator having a particular configuration of shallow quantum wells that results in reduced escape times. To provide the shallow quantum wells the effective bandgap energy of barrier layers in the modulator is chosen to be less than the sum of a longitudinal optical phonon energy and an exciton absorption energy in the modulator. U.S. Pat. No. 5,436,756 to W. H. Knox et. al. describes reducing current from photo-induced electrons in an MQW modulator by seeding the quantum well region of the modulator with non-radiative recombination centers such as protons.
A superlattice (SL) structure also comprises a series of quantum wells that are formed by a stack of quantum well layers sandwiched between barrier layers. However, in an SL, as distinguished from an MQW structure, widths of the barrier layers and heights of the quantum wells are such that wave functions of electrons and holes in quantum wells are not confined to individual quantum wells. There is substantial tunneling of electrons and holes between quantum wells. The wave functions in the quantum wells are relatively strongly coupled and in effect form extended wave functions that span substantially the full height of the stack of quantum well layers and have energies that form bands of allowed energies. When an electric field is applied to the SL perpendicular to the layers in the SL, coupling of wave functions between quantum wells is reduced and energies of allowed wave functions of electrons and holes in a same quantum well layer are displaced away from each other. The displacement is a result of narrowing of the widths of the energy bands defined by the allowed wave functions. As a result, a minimum amount of energy required to transfer an electron from the valence band of the quantum well layers to the conduction band in the quantum well layers is increased and the absorption spectrum of the MQW is blue shifted.
Optical modulators comprising SL structures are described in U.S. Pat. No. 5,194,983 to P. Voisin, the disclosure of which is incorporated herein be reference. Absorption spectra showing blue shifts for an SL structure having 4 nanometers thick layers are shown in the patent for different electric fields applied to the SL structure.
SLs in which absorption spectra of the SLs are red shifted are also possible. In “red shift” MQW modulators, red shifts that are used to modulate light are provided by changing energy differences of transitions, “direct transitions”, that occur between allowed electron and hole energy states in a same quantum well layer. “Oblique” transitions between allowed states of electrons and holes in quantum wells in adjacent quantum well layers of an SL provide an absorption spectrum that is red shifted by application of an electric field. However, red shifts provided by oblique transitions generally result in changes in absorption coefficients for light that are substantially smaller than changes in absorption coefficients provided by “direct” red shifts. In order to provide desired On/Off ratios, SL modulators that use oblique transitions to modulate light must generally provide relatively long path lengths for the light through quantum well layers of the SL. U.S. Pat. No. 5,073,809 to E. Bigan et. al., the disclosure of which is incorporated herein by reference, describes a “red shift SL” modulator in which a quantum well layer functions as a core of a waveguide having sufficient length to provide a suitable On/Off ratio.
Because barrier layers are relatively thin in SLs, escape times for photo-induced electrons and holes in SLs are relatively short and SLs are not as sensitive to escape times as are MQW modulators. However, changes in absorption coefficients for light at an operating wavelength of an SL are substantially smaller than changes in absorption coefficients achievable for light at an operating wavelength of an MQW modulator. On/Off transmission ratios for SLs are therefore generally substantially less than On/Off transmission ratios achievable with MQW modulators.